System for controlling cement flow in a well

ABSTRACT

A method for preventing undesired cement flow in a well containing a tubing string, comprises: determining a desired pressure drop for a desired flow rate at a desired location in the tubing string, determining a flow area that will cause the desired pressure drop, and installing in the tubing string a choke having at least one port, wherein the total area of the port or ports equals the desired flow area. The choke may include at least two ports, one of which may include a least one rupture disk disposed selected to rupture when the pressure drop across the choke exceeds a predetermined value. The choke may be below a float collar in the well. The invention allows the use of foamed cement and in particular foamed cement having a density that is less than the lowest density that could have been used if the choke were not in place.

RELATED CASES

This application claims priority from U.S. application Ser. No.61/413,676, filed 15 Nov. 2010, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The invention relates to a system and method for preventing undesiredcement flow in a well containing a tubing string, including installingin the tubing string a choke having at least one port with a desiredflow area.

BACKGROUND OF THE INVENTION

Subsequent to drilling a borehole of an oil or gas well, casing or lineris run into the well and a cement slurry is placed in the annulusbetween the outside of the casing and the borehole wall.

Referring initially to FIG. 1, a typical arrangement will include aborehole 10 into which a casing or liner string 12 extends, forming anannulus 11 between the borehole wall and the outside of casing 12. Insome instances string 12 will hang from a second tubing string 16 thatis placed higher in the well. In such instances, a hanger 18 supportsstring 12. String 12 is typically provided with a float collar 14 nearits lower end. When it is desired to cement string 12 in the well, alanding string 17 is positioned such that its lower end is within theupper end of string 12. A packer 15 seals the annulus 19 between string17 and string 12. As illustrated in FIG. 1, at least four pressure zonesare created in such a system, namely a first zone 30 within string 17,second zone 32 within string 12 and above collar 14, a third zone 34within string 12 and below collar 14, and a fourth zone 36 betweenstring 12 and the wellbore.

When it is desired to perform a cementing operation, a calculation ismade to determine the volume of cement that will be required in order tofully occupy the desired space. Once the pre-determined amount of cementhas been pumped into the well, a second fluid, often drilling fluid orwater, is pumped behind the cement to displace the cement out the bottomof the casing and up into the annulus between the casing and boreholewall. The cement slurry is usually raised to a point above the uppermostsection of the formations to be isolated and may be raised into theannulus between the casing string being cemented and a previouslycemented casing.

A positive pressure difference between zone 34 and zone 36 is requiredin order to place the cement in the annulus. When the cement is notflowing, the pressure in zone 34 is the hydrostatic pressure of thecement column above zone 34. When the cement is flowing, i.e. beingpumped into the well, the pressure in zone 34 is the hydrostaticpressure of the cement column above zone 34 less the pressure dropattributable to friction and any pressure drop across collar 14.

It has been found that in some instances, the pressure in zone 34 is somuch greater than the pressure in zone 36 that the cement flows fromzone 34 to zone 36 more rapidly than desired. This may occur, forinstance, when the cement is significantly denser than the fluid inannulus 11. This pressure differential then causes fluid to moveuncontrollably faster, resulting in excessive annular frictionalpressure drop and uncertainty in fluid location within the wellbore.

In addition, a differential pressure typically exists between the fluidcolumn in the annulus (zone 36) and the pore pressure of the exposedformation. The hydrostatic pressure in zone 36 immediately after thecement is placed is typically designed to be higher than the porepressure of exposed formations in order to prevent flow of formationfluids into the annulus. It is also desirable, however, to ensure thatthe pressure in zone 36 is less than the fracturing pressure of theexposed formation, since otherwise the formation would fracture and thecement slurry would flow into the formation rather than filling up theannulus around the casing.

To reduce the likelihood of losses into the formation, lightweightcements have been developed. Low-density cement slurries can be providedby including in the slurry a low-density aggregate such as graphite orhollow spheres, by diluting the cement with additional water, or bycreating a foam from the slurry. By way of example only, typical oil- orgas-well slurries can have densities of 1380 kg/m³ to 2280 kg/m³ (11.5lbm/gal to 19.0 lbm/gal) while fluids used in specialized techniques,such as foamed cementing and particle-size distribution cementing, canextend this range to 840 kg/m³ to 2760 kg/m³ (7 lbm/gal to 23 lbm/gal).

When foamed cement is being used, the rate of flow from zone 34 to zone36 may be such that an undesirable pressure drop occurs in zone 32 orzone 30. Such a pressure drop can cause gas bubbles in the cement tocoalesence into larger and less stable bubbles. Extremely large bubbles,or pockets of gas in the cement are undesirable for many reasons,including their effect on cement integrity. Gas pockets can alsoincrease the certainty of cement placement within a well.

Additionally, this rate of flow resulting the aforementioned excessivepressure in zone 36 may be uncontrollable, not only resulting inexcessive pressure in zone 36 but also very low pressure in zone 30.Commonly referred to as a U-tubing effect, the outcome can beuncontrolled placement of the cement slurry and subsequent formationfailure in zone 36 or unacceptably low pressure in zone 30. The purposeof this invention is to provide a means to induce a back-pressure insidethe casing, thus limiting or eliminating the U-tube effect.

By controlling the cement flow rate and thus the pressure in zones 30and 32, the flow of cement into the annulus can be controlled, resultingin turn in better control over the placement and quality of the cement,especially that of foamed cements. There remains a need for a system orapparatus that would provide such control.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the invention there isprovided for controlling the cement pressure. Thus the present inventionalso provides means for controlling the flow of cement into the annulus,resulting in turn in better control over the placement and quality ofthe cement.

As used in this specification and claims the following terms shall havethe following meanings:

“Above,” “upper,” and “uphole,” shall all refer to objects or locationsthat are relatively closer to the surface than a second object orlocation.

In certain embodiments, the present invention provides a method forpreventing undesired cement flow in a well containing a tubing string,comprising the steps of: a) determining a desired pressure drop for adesired flow rate at a desired location in the tubing string; b)determining a flow area that will cause the desired pressure drop; andc) installing in the tubing string a choke having at least one port,wherein the total cross-sectional area of the port or ports equals thedesired flow area.

The choke may includes at least two ports and may further include atleast one rupture disk disposed in a port and at least one port that isunobstructed. The rupture disk is selected to rupture when the pressuredrop across the choke exceeds a predetermined value.

The choke may be positioned in the well below a float collar or thelike. The method may further include the step of cementing the wellusing foamed cement and, if desired, the foamed cement may have adensity that is less than the lowest density that could have been usedif the choke were not in place.

The desired pressure drop across the choke may be in the range ofbetween 100 (0.69 MPa) and 1000 psi (6.9 MPa) and in some embodiments atleast one port can be actuated from a closed mode to an open mode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed understanding of the invention, reference is made tothe accompanying wherein:

FIG. 1 is a schematic illustration of a conventional cementingarrangement; and

FIG. 2 is a schematic illustration of a cementing arrangement configuredin accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 2, the strings 12, 16 and 17, packer 15, and floatcollar 14 are configured essentially as set out above with respect toFIG. 1. In addition to float collar 14 near the bottom of string 12, achoke 20 is included in string 12. Choke 20 creates an additionalpressure zone, 33, which is below float collar 14 and above choke 20.Choke 20 comprises a body 20 that is affixed to the inner surface ofstring 12 and includes at least one and preferably a plurality oflongitudinal bores or ports 24 extending therethrough. As discussedbelow, the total area of bore(s) 24 is preferably selectively determinedbased on the desired pressure drop across choke 20 and the anticipatedviscosity, density, pressure and flow rate of the cement.

As is known in the art, a constriction in the flow of an inviscid,incompressible fluid will cause a pressure drop AP that is a function ofthe fluid density and the velocities of the fluid upstream of theconstriction and at the constriction, per Eq. (1):

${\Delta \; P} = {\frac{\rho}{2}\left( {v_{2}^{2} - v_{1}^{2}} \right)}$

Since the velocity is dependent on flow area for an incompressiblefluid,

$v_{2} = {v_{1}\left( \frac{A_{1}}{A_{2}} \right)}$

and αP can be expressed as function of the ratio of flow areas:

${\Delta \; P} = {\frac{\rho}{2}{v_{1}^{2}\left( {\left( {A_{1}/A_{2}} \right)^{2} - 1} \right)}}$

where π=density, v₁ and v₂ are the velocity upstream and at the choke,respectively, and A₁ is the cross-sectional area of zone 33 and A₂ isthe sum of the cross-sectional areas of bore(s) 24, respectively. Tocompensate for frictional losses and the effect of viscosity, adischarge coefficient may be included in the equation.

In preferred embodiments, choke 20 comprises a drillable plate that isinserted in the casing/liner string to be cemented in place below anyfloat collar or landing collar, so as to avoid interference with anyexisting casing/liner hardware. The diameter(s) and total area ofbore(s) 24 is selected so that induce a specific, desired back-pressurewill be induced inside the casing at the intended flow rate(s).

By way of example only, in a 9⅝ inch (24.5 cm) liner, the ID of theliner will be 8.535 inches (21.7 cm) and choke 20 may include 4 boreseach having a diameter of ¼ inches (6.4 mm). At total flow rates in therange of 3 bbl/min to 4 bbl/min this configuration would result in apressure drop of 320 psi (2.2 MPa) to 570 psi (3.9 MPa) across choke 20.

In preferred embodiments, one or more of bores 24 may include a rupturedisk 26 therein. Rupture disks may be conventional rupture disks such asare known in the art. The purpose of the rupture disks is to allowcontinued fluid flow in the event bores 24 become plugged or the inducedback-pressure in zone 33 is higher than intended. Each rupture disk canbe optionally designed to fail at various pressures. By way of exampleonly, one rupture disk may be designed to fail when the pressure in zone33 is 250 psi (1.7 MPa) above desired backpressure, a second rupturedisk may be designed to fail when the pressure in zone 33 is 500 psiabove desired backpressure. In some embodiments, it may be desirable toprovide rupture disks having a total area equal to or greater than thetotal flow area of bores 24.

By way of further example, Table 1 is provided below.

Flow rate Flow Port RD1 RD2 ΔP (bbl/min) diameter 750 psi 1,000 psi 500psi 4 ¼″ ½″ 1″ 6 5/16″ ⅝″ 1¼″ 8 ⅜″ ¾″ 1½″

In preferred embodiments, chock 20 comprises a concrete or compositebody with brass/aluminum sleeves in bores 24 for wear/erosionprotection. It will be recognized by those skilled in the art that choke20 may be constructed in any manner and of any materials that aresuitable for use in a downhole environment.

By inducing controlled back-pressure inside the casing, i.e. in zone 30,32, and 33, choke 20 reduces the U-tube effect and allows more controland placement of the cementing fluids. It also reduces the risk ofbubble coalescence during foamed cementing operations.

In addition, positive pressure indication during the cementing operationhelps ensure proper placement and displacement of fluids and ensuresbetter control over the entire pumping process, thus a better-qualitycementation of the pipe in the wellbore. In turn, the present inventionallows the use of foamed cements having a higher gas/liquid ratio thanwould otherwise be possible, enabling lower effective density cementslurries to be pumped with control.

The present invention avoids interference with subsea plug launchsystems, as well as ensuring that the backpressure is present during theentire mixing and displacement. Further, the present invention allowsfor normal cementing operations with minimum modifications to thecurrent casing design.

While the present invention has been described herein in terms ofpreferred embodiments, it will be understood that various modificationscan be made thereto without departing from the scope of the invention,which is established by the claims that follows. For example, thedimensions, configuration, orientation, materials, and number of thepresent choke that are used in a given well can vary and are limitedonly by their suitability for that well. By way of specific examples:bore(s) 24 do not need to be longitudinal or cylindrical, so long asthey extend from zone 33 to zone 34; rupture disks 26 need not bedisposed in the radially outer bore(s); and the distance between choke20 and float collar 14 can be set to any desired value or order in thepipe string. It is also envisioned that controllable embodiments ofchoke 20 can be constructed, in which one or more bore(s) 24 can beactuated to an open mode as desired, using a control mechanism or signalfrom the surface or elsewhere.

What is claimed is:
 1. A method for preventing undesired cement flow ina well containing a tubing string, comprising the steps of: a)determining a desired pressure drop for a desired flow rate at a desiredlocation in the tubing string; b) determining a flow area that willcause the desired pressure drop; and c) installing in the tubing stringa choke having at least one port, wherein the total cross-sectional areaof the port or ports equals the desired flow area.
 2. The methodaccording to claim 1 wherein the choke includes at least two ports. 3.The method according to claim 1 wherein the choke includes at least onerupture disk disposed in a port and at least one port that isunobstructed.
 4. The method according to claim 3 wherein the rupturedisk is selected to rupture when the pressure drop across the chokeexceeds a predetermined value.
 5. The method according to claim 1wherein step c) comprises installing the choke below a float collar inthe well.
 6. The method according to claim 1, further including the stepof: d) cementing the well using foamed cement.
 7. The method accordingto claim 6 wherein the foamed cement has a density that is less than thelowest density that could have been used if the choke were not in place.8. The method according to claim 1 wherein the desired pressure drop isin the range of between 100 (0.69 MPa) and 1000 psi (6.9 MPa).
 9. Themethod according to claim 1 wherein at least one port can be actuatedfrom a closed mode to an open mode.